IMAP-IDLE support for Mail.app in Mac OSX 10.4 (Tiger): IMAP-IDLE-1.07.zip. There is no version that works in 10.5 (Leopard).

• Download the mailbundle and unpack the zip file (double-click in Finder)
• Move the mailbundle file to ~/Library/Mail/Bundles. You may need to create the directory "Bundles" in that folder. (The "~" means "Your home directory".)
• Check that you have installed the BSD subsystem package when installing MacOS X. If you didn't, the defaults command will not be available.
• In Terminal, type the following two commands (or copy/paste them in one at a time):
  • defaults write com.apple.mail EnableBundles -bool true
  • defaults write com.apple.mail BundleCompatibilityVersion 2